The effects of precipitation on the moist potential vorticity substance are investigated by analyzing the MPVS with precipitation mass forcing. It is found that torrential rain can cause an obvious MPVS anomaly which, combined with its impermeability, may be used to detect the location and shift of the rain band. A torrential rainfall case over the Yangtze River Basin on June 1999 is illustrated. The case study shows that the rain band coincides with the MPVS anomaly region, and therefore this anomaly is a good dynamical signal for indicating the location and shift of the Meiyu front torrential rain band.